In the field of dentistry, it is standard practice to create a cast, or model, of the maxillary arch of a patient, and a corresponding cast, or model, of the corresponding mandibular arch of the patient. It is standard to relate the models to each other with respect to a simulated axis of rotation of the condyle of the patient's lower jaw; "relate" meaning to position with respect to each other. Both models are typically held by a device known in the art as an articulator. An articulator has an upper arm hinged to a lower arm about a main axis of the articulator. The maxillary model is attached to the upper arm, and the mandibular model is attached to the lower arm. The axis of the articulator constitutes the mentioned simulated axis of rotation of the condyle of the patient's lower jaw.
Standard practice has been to use a so-called face bow for positioning the maxillary model with respect to the simulated axis of rotation. Such a device is shown, for instance, in U.S. Pat. No. 4,668,189. In FIG. 1 of such patent, the face bow is shown in phantom as it is used in connection with an articulator. The present invention is particularly directed to replacing the use of a face bow entirely in the foregoing operation.
Briefly, in using a prior art face bow, a patient first bites his or her upper teeth into a bite fork of the face bow. A first type of face bow has a pair of aligned, spaced pointers that are inserted into the ear canals of the patient while the patient is biting on the bite fork of the device. The bite fork and pointers are then fixed in position with respect to each other. The face bow is removed from the patient, and then used in conjunction with an articulator for positioning a maxillary model with respect to the main axis of rotation of the articulator. The pointers of the face bow are positioned at a standard deviation of approximately 13 millimeters from the main axis of the articulator. Such standard deviation is an approximation of the actual deviation from the ear canal of a patient to his or her axis of rotation of the condyle of the patient's lower jaw.
A drawback to using the foregoing type of face bow is that the standard deviation mentioned above differs in many cases from the actual deviation from the ear canal of a patient to his or her axis of rotation of the condyle of the patient's lower jaw. This can result in inaccuracies in the relationship of the maxilla (i.e., upper jaw) to the hinge axis, which in turn may lead to discrepancies in anything produced with the articulator.
The foregoing problem is avoided by another type of face bow, known as a non-arbitrary face bow. This is because such type of face bow has pointers placed on the condyles whose location is determined by palpation. However, both the non-arbitrary type of face bow, and the first-mentioned face bow suffer from the drawback, during positioning of a face bow onto a patient's features (as described above), when the patient's upper teeth are separated form his or her lower teeth by the inherent vertical spacing of the bite fork of the face bow. Such vertical spacing typically gives rise to an inaccuracy in positioning a maxillary model with respect to the axis of the condyle of a patient's lower jaw. This is because such axis normally shifts position as a patients upper and lower teeth are spaced vertically apart. For instance, if a typical adult separates his or her upper and lower teeth, the axis of the condyle of his or her lower jaw can move down and forward, relative to a closed-bite position, by at least several millimeters.
The foregoing drawback further results in dental appliances produced from the so-positioned maxillary and mandibular models not being predictably formed in an accurate manner.